Belly full of virus

An interesting phage-related note from today:

I saw this Nature news piece/minireview about the history of phage research this afternoon. It's nothing earth-shattering, but it's quite a nice introduction if you're completely unfamiliar with bacteriophage(s) or why they're relevant to biology. It includes all the big numbers I like to use in any phage-related presentation, like the estimate of >10^31 phages on the planet* (or in the oceans, at least). The bit immediately after that jogged my memory:

“In humans, the main genetic difference between two individuals is the phages in their gut”

It's from this 2010 Nature paper by Reyes et al. I think we can interpret "main genetic difference" as "the single largest quantity of genetic material coding for entirely different products". I mean, we can already get to that point by process of elimination. Individual people are much more genetically similar than they are different, partially because massive genetic differences usually cause massive phenotype changes and are selected against, and partially because the remaining SNPs are in the minority among nucleotides.** Even if we include all the copies of the human genome in a human body (a difficult number to estimate easily, since the number of cells in an average body remains unclear but should be around 3.7 * 10^13 cells) most of them contain the same genome and are practically identical between individuals, too. That leaves the occupants of the human microbiome. Assuming we're comparing two healthy adults rather than infants or people with GI disease, their microbial gut occupants are genetically similar. There's usually a lot of Bacteroides.

All that being said, the Reyes et al. paper does a nice job of showing how different that remaining non-somatic, non-microbial component of the human body differs between otherwise related individuals.

This is what I like so much about the whole concept: you may currently contain a mix of phages not present anywhere else on the planet. If we group the similar ones into their respective pangenomes, you may contain entirely novel genetic information. Most phages code for some of the same components (a capsid, a tail of some sort, etc.) but other than that, they're pure evolution machines, like all of us are in the end. 

* Wommack KE, Colwell RR (2000) Virioplankton: viruses in aquatic ecosystems. Microbiol Mol Biol Rev 64: 69–114. Available: http://mmbr.asm.org/cgi/content/abstract/64/1/69. Accessed 14 June 2011. 

** If we estimate there are 30 million SNPs in the 3 billion base pair human genome, that's ~1% variation.